This project involves the study of phospholipid metabolism in developing animals. Radiolabeled phospholipid precursors (palmitic acid, choline, glycerol) will be given as timed injections to term newborn, young, and adult rabbits. Subcellular fractions of lung will be isolated by differential and sucrose density gradient centrifugation. The subcellular synthesis and processing of surfactant phospholipids to secretion will be defined by measuring radioactivity, phospholipid compositions, and enzymatic activity. The type of organelles in the subcellular fractions will be defined by electron microscopy. Other studies will investigate the pathways of incorporation of palmitic acid into lung phospholipids by measuring the specific activities of the palmitate in the 1-acyl and 2-acyl positions soon after isotope injection. Radiolabeled acetate will be used to generate an in vivo synthesized endogenous lung pool of radiolabeled palmitic acid. The palmitic acid derived from de novo synthesis will be compared in vivo with blood borne palmitic acid as a substrate for surfactant. The labeling of the 1-acyl and 2-acyl positions of phosphatidylcholine and saturated phosphatidylcholine by the acetate derived palmitic acid will be defined.